1. Field of the Invention
This invention involves optical systems for the detection of chemicals, in particular, to a self-locking optical cavity that generates an optical signal in response to the presence of a chemical analyte.
2. Description of the Related Art
The sensitivity and dynamic range of optical techniques make them well suited for use in chemical sensing systems. Accordingly, several sensing devices are now available that attempt to put these properties to use.
In any optical sensing system, the light source is a critical component. In choosing a light source, the designer must typically make a trade-off among the often conflicting requirements for high optical power levels, high efficiency, low cost, small size, and structural strength. One particularly advantageous development in this regard is that progress in solid state technology has led to the miniaturization of coherent light sources from meters (such as argon ion or helium-neon lasers) to microns (such as surface emitting quantum well laser diodes). For example, solid state diode lasers are now commercially available with output powers ranging from milliwatts to watts. An example of such a device is described in Parke, R., et al., "2.0 W cw, diffraction-limited operation of a monolithically integrated master oscillator power amplifier," IEEE Photon. Tech. Lett., 5, pp. 297-300, (1993).
While the wall-plug efficiency of diode lasers is high, sources that directly generate many watts of optical power often require many more watts of electrical power just to cool them. For a sensing application one must therefore balance the requirement for multiple watts of optical power with the desire for a compact and portable device that only consumes a small amount of electrical power.
One solution to this problem relies on using light trapped inside an optical cavity. An optical cavity or resonator consists of two or more mirrored surfaces arranged so that incident light may be trapped, bouncing back and forth between the mirrors. In this way, the light inside the cavity may be many orders of magnitude more intense than the incident light. This general solution is well known and has been exploited in various ways, such as for nonlinear frequency conversion (see, for example, Yariv A., Introduction to Optical Electronics, 2nd Ed., Holt, Rinehart, and Winston, N.Y., 1976, Chapter 8), and most widely for spectroscopy, such as is described in Demtroder, W., Laser Spectroscopy, springer-verlag, Berlin, 1982, pp. 390-395.
The extension of this solution to chemical sensing relies on the interaction of the intracavity light and the chemical analyte to generate an optical signal. The optical signal may usually be coherent or incoherent, and need not necessarily be at the same frequency as the intracavity light. The magnitude of the optical signal is determined by the amount of chemical analyte present and the intensity of the intracavity light. This technique has been applied to gas monitoring where the optical signal is generated through spontaneous Raman scattering. See, for example, U.S. Pat. No. 4,648,714, "Molecular Gas Analysis by Raman Scattering in Intracavity Laser Configuration," Benner, et al., 10 Mar. 1987.
In all of the previously mentioned methods the optical gain medium (such as a helium neon discharge tube) is within the optical cavity. For a typical diode laser the cavity mirrors are deposited directly on the diode gain medium itself. For some applications, however, such as frequency tuning and linewidth narrowing, one or both of the diode's facets is anti-reflection coated and the diode is operated inside an optical cavity that is defined by mirrors that are external to the diode. While a diode gain media may be operated inside such a cavity, the low damage threshold of the diode's emission facet severely limits the amount of power buildup. In other words, if the diode is placed within the cavity, one cannot allow the power to build up so much that the diode itself is damaged, yet the maximum permissible power is often too little to allow for efficient and simple sensing schemes.
To overcome this limitation while still generating a large optical field, the diode laser may be placed outside of a separate high finesse optical cavity, in which the diode laser radiation is trapped. This separate cavity is referred to below as a "build-up" cavity. Diode lasers, however, emit coherent radiation with an optical bandwidth that is much larger than that of a high finesse build-up cavity. To achieve substantial amplification of diode laser radiation in a build-up cavity the diode laser must bet forced to emit coherent radiation with a linewidth that approaches or matches that of the cavity.
There are several well-known techniques for reducing the bandwidth of diode lasers. For example, active, all-electronic frequency locking of diode lasers may be used. This technique, however, requires very fast servos, with bandwidths up to and even greater than 20 MHz, and a large degree of optical isolation of the diode laser from the cavity. Passive locking has important advantages over active all-electronic locking; for example, the necessary electronic control is greatly reduced, especially if narrow-band radiation is required, and optical isolators may not be needed.
As an alternative, substantial linewidth reduction can be achieved with optical feedback schemes. For example, Dahmani et al., in "Frequency stabilization of semiconductor lasers by resonant optical feedback," Opt. Lett., 12, pp. 876-878 (1987), reported passive optical locking of a diode laser to a build-up cavity. In this technique, light from a diode laser is directed into a build-up cavity and if the light frequency is matched to a cavity resonance frequency then the light is trapped. A portion of the trapped light is then directed back into the diode laser to act as a passive feedback mechanism, which locks the frequency of the low-finesse diode laser to that of the high-finesse build-up cavity.
Tanner et al., in "Atomic beam collimation using a laser diode with a self-locking power buildup cavity," Opt. Lett., 13, pp. 357-359 (1988), describe a self-locking, power-buildup cavity that generates a thousand-fold more light inside the cavity than was incident, but they exploit this intense intracavity light field only for optical pumping of cesium atoms. This technique, however, was recently applied to visible diode lasers, as is described by Simonsen, H. R., in "Frequency noise reduction of visible InGaAlP laser diodes by different optical feedback methods," IEEE J. Quant. Elec., 29, pp. 877-884 (1993).
A shortcoming of the systems described by Dahmani et al., Tanner et al., and Simonsen is that they all employ weak optical locking: only a very minute portion of the light in the build-up cavity is fed back to the diode laser. The disadvantage of the weak optical locking technique, however, is that it still requires careful electromechanical control of both the magnitude and phase of the light fed back to the diode laser. This is discussed, for example, in U.S. Pat. No. 4,907,237, "Optical Feedback Locking of Semiconductor Lasers," Dahmani, B., et al., 6 Mar. 1990; Hemmerich, A., et al., "Second-harmonic generation and optical stabilization of a diode laser in an external ring resonator," Opt. Lett., 15, pp 372-374 (1990); and Buch, P., and Kohns, P., "Optically self-locked semiconductor laser with servo control for feedback phase and laser current," IEEE J. Quant. Elec., 27, 1863 (1991).
A self-locking power build-up cavity has also been used specifically for nonlinear generation of intense amounts of coherent radiation. The use of low to moderate (&lt;1%) feedback from a build-up cavity to optically lock the laser diode to the cavity is described, for example, in the article by Hemmerich, et al. that is mentioned above, and also in Dixon, G. J., Tanner, C. E., and Wieman, C. E., "432-nm source based on efficient second-harmonic generation of GaAlAs diodelaser radiation in a self-locking external resonant cavity," Opt. Lett., 14, pp 731-733 (1989); and U.S. Pat. No. 4,884,276, "Optical Feedback Control in the Frequency Conversion of Laser Diode Radiation," Dixon, et al., 28 Nov. 1989.
This concept is taken further by W. Lenth and W. P. Risk in U.S. Pat. No. 5,038,352, "Laser System and Method Using a Nonlinear Crystal Resonator," 6 Aug. 1991, in which they teach that the use of an anti-reflection (AR) coated diode laser and strong (10%-50%) feedback increases the locking stability. Furthermore, W. J. Kozlovsky, together with Lenth and Risk, reported in "Resonator-enhanced frequency doubling in an extended cavity diode laser," in Proceedings of the Compact Blue-Green Lasers Topical Meeting, New Orleans, La., Optical Society of America, February 1993, p. PD2-1, how they employed strong (3%) optical feedback to an AR-coated diode laser and added a dispersive element that reflected light emitted from the build-up cavity back through the cavity and into the diode laser. The dispersive element added frequency stability.
A passive all-optical frequency locking technique for a diode laser is simpler and more stable than either all-electronic or weak-optical-feedback locking because it eliminates the need for sophisticated electronic control of the diode laser or of a light field. To ensure that the diode laser remains stably locked to the build-up cavity it is essential that the dominant optical feedback into the diode gain medium should be from the build-up cavity rather than from any other source, for example, from reflection of light from the diode laser emission facet back into the diode gain medium.
Typically, the diode emission facet is anti-reflection coated, and the whole system may be viewed as a regular solid-state laser with an intracavity etalon. The more light that is fed back to the diode laser from the build-up cavity, the higher is the reflectivity of the diode emission facet that permits stable optical locking.
of all the previously mentioned methods the only two that employ all-optical locking with large feedback are those disclosed by Lenth and Risk (U.S. Pat. No. 5,038,352), and by Kozlovsky et al. Neither of these methods, however, is well suited for sensing applications: it is essential for the stated purpose of these systems to halve the wavelength of the incident light, in particular, to turn near infra-red light into blue.
As Siegman describes in Lasers, University Science Books, Mill Valley, Calif., 1986, pp. 428-31, the power that can be generated inside an optical build-up cavity is inversely proportional to the optical loss of the cavity. The optical loss is approximately the sum of the optical losses of all the individual intracavity elements such as mirrors.
A structure that is optimal for coherent generation of radiation necessarily produces a useful amount of output radiation (see Kozlovsky and Lenth). This conversion also acts as an additional loss mechanism for the intracavity radiation and reduces the amount of power that can be generated in the optical cavity. The non-linear crystal used in systems such as Lenth's and Kozlovsky's has additional absorption losses, which act to reduce the cavity finesse; both these substantial loss mechanisms are essential for coherent generation of radiation. For intracavity chemical sensing, however, in particular for Raman gas sensing, one requires as large an intracavity power as possible, consequently, any structure that incorporates unnecessary and substantial losses is unsuitable for sensitive intracavity chemical sensing. A structure that is best suited for chemical sensing would be a build-up cavity that has as low an optical loss as practical, so that as little an amount of light as possible escapes the cavity and is thus fundamentally different from that described by Lenth and Kozlovsky.
In the context of external cavity semiconductor lasers, it is well known that anti-reflection coating of the diode output facet and strong optical feedback are essential for stable performance. See Rong-Qing, H., and Shang-Ping, T., "Improved rate equations for external cavity semiconductor lasers," IEEE J. Quant. Elec., 25, pp. 1580-1584, 1989. These laser systems have been employed for intracavity spectroscopy, see Baev, V. M., Eschner, J., Paeth, E., Shuler, R., and Toschek, P. E., "Intra-cavity spectroscopy with diode lasers," Appl. Phys. B., B55, pp. 463-477, 1992, but, as discussed above, the cavity finesse and thus intracavity power are deliberately kept low to prevent optical damage to the laser facet of the diode. These devices are therefore unsuitable for sensing applications that rely on high intracavity power.
For chemical sensing, what is needed is a compact system that combines high intracavity power with low required input power and that locks passively and optically, thereby doing away with a need for complicated and expensive locking circuitry.